Conventionally, a resin molded product, having molded portions disposed respectively on opposite sides of a rigid, thick membrane, has been manufactured in the following manner.
As shown in FIG. 3, a thick membrane 1 disposed within a cavity of molds 3 is clamped between core pins 4 and 4', and a desired resin molded product is produced by injection molding. FIG. 3(a) shows such injection molding, and FIG. 3(b) is a cross-sectional, front-elevational view of the resin molded product. Reference numeral 2 denotes the resin, and reference numeral 5 denotes a gate for supplying the resin. However, when instead of a thick membrane, a thin membrane is employed with the above conventional molding method, the thin membrane is deformed by the pressure of the injected resin. As a result, the membrane is not retained or held by the resultant molded product, as shown in FIG. 4, thus failing to provide the desired molded product. FIG. 4(a) shows an initial stage of the injection molding, and FIG. 4(b) shows an intermediate stage of such injection molding, and FIG. 4(c) is a cross-sectional, front-elevational view of the molded product. In FIG. 4, reference numeral 2 denotes the resin, reference numeral 3 denotes a mold, reference numerals 4 and 4, denote core pins, and reference numeral 5 denotes a gate.
To overcome the above difficulty, the following procedures have been proposed.
(1) As shown in FIG. 5, a thin membrane 6 is fused or heat-bonded to a molded part 10, and then another molded part 11 is secured to the molded part 10 by ultrasonic welding.
(2) Alternatively, as shown in FIG. 6, flanges of molded parts 10 and 11 are enlarged, and the two molded parts are joined together by ultrasonic welding (as indicated by an arrow), with a thin membrane 6 held therebetween.
However, procedure (1) above is costly, and procedure (2) above is not satisfactory because it results in high cost and an increased size of the parts.
An air vent device is known which is both gas permeable and water-proof, and comprises a housing of a resin and a porous membrane of polytetrafluoroethylene (PTFE) or the like bonded to the resin housing by heat welding or ultrasonic welding.
However, in the case where the porous membrane is bonded to the resin housing which is rigid, a seal material or an adhesive sealant must be used to provide an air-tight seal between the air vent device and a container when mounting the air vent device on the container.
Also, in the case where the porous membrane is bonded to the resin housing of a flexible or soft nature by heat welding or ultrasonic welding, the housing is susceptible to deformation so that a gap tends to develop between the porous membrane and the housing, thus failing to provide a water-proof seal.